I think Nashville is a prime location for an expansion team.
A number of cities have been mentioned for expansion teams. Nashville is often mentioned. So are Montreal, Charlotte, Austin , San Antonio, Salt Lake City, and even longshots such as Mexico City, Vancouver, and Portland. Perhaps now Oakland will be added to the list.
Rob Manfred has said they would like to add two teams, once the stadium situations for the Athletics and Tampa Bay Rays are settled. Adding 2 teams would get major league baseball to 32 teams, which is supposed to be much easier to deal with, as far as scheduling is concerned.